1. Field of the Invention
The present invention relates to an image reading apparatus provided with a device that optically reads an original, such as a scanner installed in a copier or a fax machine, or a single flat-bed scanner.
2. Description of the Related Art
In a conventional color scanner, an RGB 3-line sensor is adopted in order to read a color original. This 3-line sensor is a sensor that receives light and convert that received light to an electrical signal, such as a CCD (Charge Coupled Device) or CIS (Contact Image Sensor). Further, by applying filters of the three primary colors of light, R (Red), G (Green), and B (Blue), for example, and combining data for each pixel, which is an electrical signal for the corresponding color gamut, the electrical signals are converted to a color image.
In an MFP (Multi Function Printer), in a color reading mode in which an image of a color original is printed in color to an output paper, or transferred to a host such as a personal computer as color image data, the original image is read using all of the sensors R, G, and B. On the other hand, there is also a monochrome reading mode in which an image of a color original is printed in monochrome to an output paper, or transferred to a fax machine or a host such as a personal computer as monochrome image data. In the monochrome reading mode, as shown in FIG. 8, image data via the G (green) filter, which has the highest electrical signal output from a sensor among R (red) 803, G (green) 801, and B (blue) 802, is adopted and converted to a monochrome image. FIG. 8 shows the output signals that are output from RGB line sensors. In FIG. 8, the horizontal axis indicates the number of pixels in a main scanning direction, and the vertical axis indicates the line sensor output signal. Reference numerals 801 to 803 respectively indicate the output of a G (green) sensor, the output of a B (blue) sensor, and the output of an R (red) sensor. The image data with the highest electrical signal output is adopted because this is advantageous for the S/N ratio of the image, and as a result a high quality image is obtained.
In this way, in many MFPs, the same sensors are often adopted for the color mode in which a color original is read as a color image and the monochrome mode in which a color original is read as a monochrome image. On the other hand, when reading a monochrome image, although high image quality with smooth tonality is also sought, often there are greater demands for high productivity.
Consequently, in Japanese Patent Laid-Open No. 2001-144900, technology is proposed relating to a 4-line line sensor in which a color image is read with an RGB 3-line color sensor, and a monochrome line sensor having higher productivity is also provided. Japanese Patent Laid-Open No. 2001-144900 describes a control method for reading a color image with the RGB 3-line sensor, and reading a monochrome image with the 1-line sensor for the monochrome mode. Also, Japanese Patent Laid-Open No. 2003-274115 proposes a method for, when the horizontal transfer speed of the monochrome line sensor is made faster than that of the color line sensor, avoiding the introduction of noise from the monochrome line sensor, which has a higher horizontal transfer speed, into the color line sensor.
However, there are the problems described below in the above conventional technology. First, the color line sensor and the monochrome line sensor will be described in detail. Light is illuminated on the RGB 3-line sensor for color reading via each of the R filter, the G filter, and the B filter. Accordingly, if the light amount and the horizontal transfer speed are the same, the output level from the line sensor is higher for the monochrome sensor, which does not employ a filter. FIG. 9 shows the output signals that are output from the RGB line sensors and the monochrome line sensor. In FIG. 9, the horizontal axis indicates the number of pixels in the main scanning direction, and the vertical axis indicates the line sensor output signal. Reference numerals 900 to 903 respectively indicate the output of a BW (monochrome) sensor, the output of a G (green) sensor, the output of a B (blue) sensor, and the output of an R (red) sensor. As shown in FIG. 9, the relationship of the output levels is BW 900>G 901>B 902>R 903.
Accordingly, even assuming that the horizontal transfer speed of the monochrome line sensor without a filter has been made faster than the horizontal transfer speed of the color 3-line sensor, the output level from each sensor when reading a white reference plate satisfies image tonality. FIG. 10 shows the output signals that are output from the RGB line sensors and the monochrome line sensor. In FIG. 10, the horizontal axis indicates the number of pixels in the main scanning direction, and the vertical axis indicates the line sensor output signal. Reference numerals 1000 to 1003 respectively indicate the output of a BW (monochrome) sensor, the output of a G (green) sensor, the output of a B (blue) sensor, and the output of an R (red) sensor. Unlike FIG. 9, FIG. 10 shows the result of making the horizontal transfer speed of the monochrome line sensor faster than that of the RGB line sensor. This will be described with reference to FIG. 11.
FIG. 11 shows the relationship between a light amount accumulation period and the output level per pixel for the RGB line sensors and the monochrome line sensor. The horizontal axis indicates the light amount accumulation period for each line sensor, and the vertical axis indicates the output level per pixel for each line sensor. Reference numeral 1101 indicates an output curve for the monochrome line sensor, reference numeral 1102 indicates an output curve for the G line sensor, reference numeral 1103 indicates an output curve for the B line sensor, and reference numeral 1104 indicates an output curve for the R line sensor.
The period indicated by Trgb is the period of accumulation in 1 pixel at the horizontal transfer speed of the color (RGB) line sensors, and the output level relationship in this case is the relationship shown in FIG. 9. However, when the horizontal transfer speed of the monochrome line sensor is made faster than that of the color line sensors in order to increase productivity, the accumulation period becomes the period indicated by Tbw. The output level in this case is indicated by BW (Tbw), and the relationship of this output level with G (Trgb), B (Trgb), and R (Trgb), which are the output levels in the accumulation period Trgb of the color line sensors, is the output level relationship as shown in FIG. 10, indicated for one line in the main scanning direction.
That is, as shown in FIGS. 9 to 11, in order to obtain a predetermined image output level, it is possible to make the horizontal transfer speed faster for reading with a monochrome line sensor than for reading with a color 3-line sensor. The time needed for consecutively reading a plurality of pages can be less in the case of original image reading in the monochrome mode employing the monochrome line sensor than when reading with the color 3-line sensor in the color mode, and so it is possible to increase productivity.
However, when the scanner is driven for a long time, the lamp progressively deteriorates. When deterioration progresses due to such driving for a long time and the light amount of the lamp decreases, in the monochrome mode employing the monochrome line sensor with a transfer clock set to a high speed, the problem occurs that the light amount is insufficient, and so image quality decreases.
In such a case, the output level from the monochrome line sensor can be ensured by setting a slower monochrome horizontal transfer clock so as to be the same as the color horizontal transfer clock, as shown in FIG. 11. However, in a state in which the light amount has decreased, it is necessary to perform readjustment that takes a long time, such as gain adjustment that amplifies the output of the line sensor or offset adjustment for determining the black level. In other words, because an adjustment operation is necessary, if in the midst of a job, it is necessary to secure time between originals and thus decrease productivity, and if prior to starting a job, it is necessary to delay a first scan in which the first page of an original is read.